Redux Toolkit：createSlice と configureStore

npm install @reduxjs/toolkit react-redux

import { createSlice } from '@reduxjs/toolkit';

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: state => { state.value++; },     // Immer: mutate directly
    decrement: state => { state.value--; },
    reset: state => { state.value = 0; },
    incrementBy: (state, action) => { state.value += action.payload; }
  }
});

export const { increment, decrement, reset, incrementBy } = counterSlice.actions;
export default counterSlice.reducer;

import { configureStore } from '@reduxjs/toolkit';
import counterReducer from './counterSlice';
import cartReducer from './cartSlice';

export const store = configureStore({
  reducer: {
    counter: counterReducer,
    cart: cartReducer,
  }
});

// TypeScript types:
export type RootState = ReturnType<typeof store.getState>;
export type AppDispatch = typeof store.dispatch;

import { Provider } from 'react-redux';
import { store } from './store';

ReactDOM.createRoot(document.getElementById('root')!).render(
  <Provider store={store}>
    <App />
  </Provider>
);

import { useSelector } from 'react-redux';
import type { RootState } from './store';

function Counter() {
  const count = useSelector((state: RootState) => state.counter.value);
  return <p>Count: {count}</p>;
}

import { useDispatch } from 'react-redux';
import { increment, incrementBy } from './counterSlice';

function Controls() {
  const dispatch = useDispatch();
  return (
    <div>
      <button onClick={() => dispatch(increment())}>+1</button>
      <button onClick={() => dispatch(incrementBy(5))}>+5</button>
    </div>
  );
}

import { createAsyncThunk } from '@reduxjs/toolkit';

export const fetchUser = createAsyncThunk(
  'user/fetchById',
  async (userId: string) => {
    const res = await fetch(`/api/users/${userId}`);
    return res.json(); // becomes action.payload on fulfilled
  }
);

// In slice:
extraReducers: (builder) => {
  builder
    .addCase(fetchUser.pending, state => { state.loading = true; })
    .addCase(fetchUser.fulfilled, (state, action) => {
      state.loading = false;
      state.user = action.payload;
    });
}

import { createApi, fetchBaseQuery } from '@reduxjs/toolkit/query/react';

export const api = createApi({
  baseQuery: fetchBaseQuery({ baseUrl: '/api' }),
  endpoints: (build) => ({
    getUsers: build.query<User[], void>({ query: () => 'users' }),
  })
});

// Generated hook:
const { data, isLoading, error } = api.useGetUsersQuery();